SENSITIVITY OF TIDAL BED STRESS DISTRIBUTIONS, NEAR-BED CURRENTS, OVERTIDES, AND TIDAL RESIDUALS TO FRICTIONAL EFFECTS IN THE EASTERN IRISHSEA

A three-dimensional nonlinear high-resolution hydrodynamic model of th e eastern Irish Sea is used to examine the influence of near-bed visco sity and bottom friction upon the M(2) tide and overtides in the regio n. Initially, bed stress in the model is related to the depth-mean cur rent, the formulation used in a ''classic'' two-dimensional model. The resulting hybrid ''two-dimensional/three-dimensional model'' enables changes in current profile due to variations in eddy viscosity to be e xamined independently of those due to changes in bed stress. Also, res ults from the model are compared with those from a three-dimensional m odel with bed stress related to bottom current. Computed tidal elevati ons and surface currents are found to be fairly insensitive to the par ameterization of bottom stress. However, tidal current profiles in the near-bed region and maximum bed stress are found to be sensitive to v ariations in near-bed viscosity and friction coefficient. Although red ucing eddy viscosity in the near-bed region and increasing bottom fric tional coefficient makes little difference to tidal elevations and nea rsurface tidal currents, it is essential to accurately reproduce the b ed currents and associated bed stress. Higher harmonics of the tide (t he overtides) computed with the model show significant spatial variabi lity in the region, with near-bed currents being influenced by frictio nal effects. Tidal residual flows also show significant spatial variab ility with near-bed hows being influenced by friction.